Every lens has a "sweet" spot - an aperture setting where the various aberrations inherent in real world lens design are at their minimum total effect, and diffraction limiting hasn't started having a large effect.
For most enlarging lenses, that "sweet" spot setting is about two stops down from maximum opening.
Sometimes however, that "sweet" spot opening either lets too much light through, or lets too little light through, to give you practical enlarging times, so you need to adjust the aperture away from it.
In addition, you generally want to compose and focus on the easel with as much light as possible, in order to see best what you are doing. Then you stop down to your printing aperture.
Finally, smaller apertures will give you greater depth of focus (helps slightly with curly negatives) as well as greater depth of field (enables adjusting for converging parallels and helps with curly printing paper).
Ahhhhh *penny drops* and the longer the lens, the bigger I can make the print? As I have a 50mm and and an 80mm lens.